Recent advancements in composite structures have produced materials of sufficient strength to permit their use in aircraft structures. The composite structures are much lighter in weight than metal, making their use preferred in an aircraft structure. Exposed aircraft surfaces, including the tail, rudder, and wing portions, may include a composite structure.
The composite structure often includes a surface region made of an electrically insulated material and various types of inner region materials which are electrically conductive. Static charge builds up on the surface of the aircraft structure as it flies through the air. Often, the aircraft flies through clouds, winds or heavy storms, which places large static charges on various parts of the aircraft structure. The static charges build up on the insulating surface until the charge density is sufficiently high to permit discharge to a different part of the aircraft. This creates sparks or electrical arcing from the insulating surface region to the discharge location. The discharge point may be metal screws, sheets of metal, engine mounts or other portions of the aircraft. A spark of this nature is extremely hot and creates high temperature heating of a localized area in the spark region. A graphite composite structure has very good fatigue properties, but the epoxy part of the composite is susceptible to damage by very high temperatures, such as those produced by an arc. Graphite tends to conduct better at very high temperatures than metal, whereas metal conducts better than graphite at room temperature. At lower temperatures metal conducts better than the composite, the resistance being lower. When a spark begins to occur and localized heating begins to take place, the composite heats up and conducts much better than metal, thus providing a lower resistance current path for electricity, which creates an even greater heating effect in the composite structure. Similar problems occur from lightning strikes.
A further safety problem may occur when an electrical device such as a pump is attached to a composite structure. A fuse is connected in series with various parts of the pump to cut off the supply of electric current should it become too high. A current that would trip the fuse is referred to as fault current. Under normal circumstances when a fault current exits the fuse will trip and protect the pump and prevent a fire from starting within the electrical wiring. If the pump is connected by a metal fastener to the composite structure it is possible that arcing or other heating current flow may cause heating at the graphite thus making it a better conductor than metal at this temperature. The fault current would then begin to flow in the graphite composite structure and bypass the fuse circuit completely. Significant current could flow to the pump bypassing the fuse causing a fire or destruction of the pump. This is even more likely if a lightning strike occurs.
Previous patents deal with the problem of static charge buildup on aircraft structures and attempt various solutions to these problems. Pat. No. 2,250,280, to Starbird, describes an electrical bonding member having protrusions, the bonding member being placed between two sheets of metal to insure better contact between the metal. Pat. No. 3,906,308, to Amason et al., describes a lightning protection system which provides metal strips affixed to the dielectric surface and grounded to provide dwell points for lightning current. Pat. No. 3,989,984 teaches use of an outer metallic layer formed from sprayed metal and ground connection to the metallic layer, including a wire screen fused with sprayed metal onto the dielectric layer to protect the composite structure and provide conductive areas for transfer of lightning current.
Pat, Nos. 4,502,092 and 4,479,163, to Bannink, Jr. et al., describe lightning protection systems for graphite composite aircraft skins. The composite structure disclosed in these patents is similar to that of the present invention; however, the approach taught by Bannink, Jr. et al. is to use an insulating layer 42 to electrically insulate the fastener 32 from the composite structure.
Pat. No. 4,628,402, to Covey, deals with the same composite structure and electrical charge-carrying problem as the present invention; however, it solves this problem by placing a larger dielectric layer over the metal fastener and then a conductive region on top of the composite structure spaced a distance away from the fastener to increase the graphite area through which the current flows to thus lower the temperature and the current through a small area in the graphite material.
Pat. No. 4,630,168, to Hunt, deals with a similar problem in similar composite structure by covering the top of the metallic fastener with an insulating layer.
Pat. No. 4,681,497, to Berecz, teaches insulating a metal bolt with a composite structure made partially of insulating resins and conducting fibers. According to Berecz, the head of the screw is electrically insulated from the surface, but the lower portions of the fastener are electrically coupled to portions of the composite strip.
Pat. No. 2,186,482 to Frank teaches use of a copper sheath to bind two steel cores together for use as a ground rod to provide good coupling between the segments of the steel core. Pat. No. 2,181,835 to Place teaches use of a coating composition for screw-threaded elements.
While these patents discuss problems similar to those solved by the present invention, many of the solutions are not cost-effective for use on aircraft structures or do not provide the full lightning and static discharge protection needed for an aircraft. Pat. Nos. 4,628,402; 4,630,168 and 4,681,497 are cited but are not admitted as prior art to the present invention as described herein.